I am looking for a book which tells the story of two worlds.
One of the worlds is very small and the other is very large (relatively to each other). The large one was I think populated with humans.
They try to communicate but the time scale in the two worlds is drastically different: a short time on the large one is orders of magnitude longer on the other one. This means that whatever is passed from one to the other will span generations.
The small world was dying I think.
There was a second book which describes the second phase of their interactions, after both sides understood the time rate difference and its implications.